In the technology of process measurements and automation, for the measurement of physical parameters of a fluid flowing in a pipeline, parameters such as e.g. mass flow rate, density and/or viscosity, it is common to use such measuring devices, which, by means of a vibratory measurement pickup inserted into the course of the pipeline conducting the fluid and traversed by the fluid during operation, and a measuring and operating circuit connected thereto, create reaction forces in the fluid, forces such as e.g. Coriolis forces related to the mass flow rate, inertial forces related to the density, and frictional forces related to the viscosity, etc., and produce, derived therefrom, one or more measurement signals representing the current mass flow rate, the current viscosity and/or the current density of the fluid. Vibratory measurement pickups of this type are described e.g. in WO-A 03/076880, WO-A 02/37063, WO-A 01/33174, WO-A 00/57141, WO-A 99/39164, WO-A 98/07009, WO-A 95/16897, WO-A 88/03261, US 2003/0208325, U.S. Pat. No. B 6,513,393, U.S. Pat. No. B 6,505,519, U.S. Pat. No. 6,006,609, U.S. Pat. No. 5,869,770, U.S. Pat. No. 5,796,011, U.S. Pat. No. 5,602,346, U.S. Pat. No. 5,301,557, U.S. Pat. No. 5,259,250, U.S. Pat. No. 5,218,873, U.S. Pat. No. 5,069,074, U.S. Pat. No. 5,029,482, U.S. Pat. No. 4,876,898, U.S. Pat. No. 4,733,569, U.S. Pat. No. 4,660,421, U.S. Pat. No. 4,524,610, U.S. Pat. No. 4,491,025, U.S. Pat. No. 4,187,721, EP-A 553 939, EP-A 1 001 254 or EP-A 1 281 938.
For guiding the fluid, the measurement pickups include always at least one measuring tube, which is held, for example, in a tubular or box-shaped support frame. The measuring tube has a curved or straight tube segment, which is caused to vibrate—driven by an electromechanical exciter arrangement—during operation for producing the above-mentioned reaction forces. For registering vibrations, particularly vibrations at the inlet and outlet ends, of the tube segment, the measurement pickups additionally have electrophysical, sensor arrangements reacting to movements of the tube segment. In the case of Coriolis mass flow meters for application to a medium flowing in a pipeline, the measuring of the mass flow rate is accomplished, for example, by allowing the medium to flow through the measuring tube interposed in the pipeline and oscillating the tube during operation, whereby the medium experiences Coriolis forces. These forces, in turn, effect that the inlet and outlet regions of the measuring tube oscillate with phases which are shifted with respect to one another. The size of this phase shift serves as a measure for the mass flow rate. Then the oscillations of the measuring tube are registered by means of two oscillation sensors of the above-mentioned sensor arrangement separated from one another along the length of the measuring tube and are transformed into oscillation measurement signals, from whose mutual phase difference the mass flow rate is derived.
Already the above-referenced U.S. Pat. No. 4,187,721 mentions that also the instantaneous density of the flowing medium is usually measurable with Coriolis mass flow measuring devices, and, indeed, on the basis of a frequency of at least one of the oscillation measurement signals delivered by the sensor arrangement. Moreover, usually a temperature of the medium is also measured directly, in suitable manner, for instance by means of a temperature sensor arranged on the measuring tube. It can thus be assumed, without more, that, even when not expressly described, modern Coriolis mass-flow measuring devices enable measurement also of density and temperature of the medium, especially considering that these measurements can, in any case, always be used for compensation of measurement errors resulting from fluctuating fluid density; see, in this connection, especially the already mentioned U.S. Pat. No. 5,602,346, as well as WO-A 02/37063, WO-A 99/39164, or, also, WO-A 00/36379.
In the application of vibratory, measurement pickups, it has, however, been found, that, in the case of inhomogeneous media, especially fluids of two or more phases, the oscillation measuring signals derived from the oscillations of the measuring tube, especially also the mentioned phase shift, are subject to fluctuations to a considerable degree, in spite of keeping viscosity and density of the separate phases, as well as the mass flow rate, practically constant and/or appropriately taking them into consideration, such that, without remedial measures, the signals can become completely unusable for measuring the desired physical parameter. Such inhomogeneous media can, for example, be liquids, into which, as e.g. practically unavoidable in dosing- or bottling-processes, gas, especially air, present in the pipeline, is entrained, or from which a dissolved fluid, e.g. carbon dioxide, outgases and leads to foam formation. Another example of such inhomogeneous media is wet, or saturated, steam.
Already in U.S. Pat. No. 4,524,610, a possible cause of this problem for the operation of vibratory measurement pickups is indicated, namely the circumstance that inhomogeneities, such as gas bubbles brought into the measuring tube by the fluid, deposit on its inner wall and, so, can influence the oscillation to a considerable degree. For avoiding the problem, it is also proposed to install the measurement pickup such that the straight measuring tube extends essentially vertically, in order to prevent, as much as possible, an attachment of such interfering, especially gaseous, inhomogeneities. This is, however, a very special solution which cannot always be realized, without more, in industrial process measurement technology. On the one hand, the pipeline, into which the measurement pickup is to be inserted namely for this case, must, on occasion, be fitted to the measurement pickup, and not the reverse, a fact which can be rather difficult to explain to the user. On the other hand, the measuring tubes can, as already mentioned, be curved, so that the problem then cannot be solved by an adjustment of orientation in the installation. It has also been found, in this connection, that the mentioned corruptions of the measurement signal cannot really be significantly decreased by the use of a vertically installed, straight measuring tube. Moreover, the observed fluctuations of the so-produced measurement signal in the case of flowing fluid can, in any event, not be prevented in this manner.
Similar causes and their effects on the accuracy of measurement in the determining of mass flow rate are discussed, for example, also in JP-A 10-281846, WO-A 03/076880, along with U.S. Pat. Nos. 5,259,250, 5,029,482 or 6,505,519. While a flow, respectively fluid, conditioning preceding the actual flow measurement is proposed in WO-A 03/076880 for lessening the measurement errors associated with fluids of two or more phases, JP-A 10-281846 and, also, U.S. Pat. No. 6,505,519, for example, each describe a correcting of the flow measurement, especially mass flow rate measurement, based on the oscillation measurement signals, for example using pre-trained, possibly even adaptive, classifiers for the oscillation measurement signals. The classifiers can be constructed, for example, in the form of a Kohonen map or a neural network, and can perform the correction either on the basis of a few parameters measured during operation, especially the mass flow rate and the density, along with further characteristics derived therefrom, or also by using an interval of the oscillation measurement signals encompassing one or more oscillation periods.
The use of such classifiers has, for example, the advantage, that, in comparison to conventional Coriolis mass flow/density meters, no, or only very slight, changes have to be made at the measurement pickup, be it with respect to the mechanical construction, the exciter arrangement, or the operating circuit controlling such, which are adjusted to accommodate the special application.
However, there is a significant disadvantage of such classifiers, among other things, in that, compared to conventional Coriolis mass flow measuring devices, considerable changes are required in the realm of measured value production, especially as regards the analog-to-digital converters and the microprocessors which are used. Thus, as described in the U.S. Pat. No. 6,505,519, such signal evaluation requires, for example in the digitizing of the oscillation measurement signals, which can have an oscillation frequency of around 80 Hz, a sampling rate of about 55 kHz, or more, in order to achieve sufficient accuracy. Said differently, the oscillation measurement signals must be sampled using a sampling ratio significantly above 600:1. On top of this, also the firmware stored and executed in the digital measuring circuit becomes correspondingly complex.
An additional disadvantage of such classifiers is to be seen in the fact that they must be trained and correspondingly validated for the measuring conditions actually present during operation of the measurement pickup, be it the conditions of installation, the fluid to be measured, and its usually variable properties, or other factors affecting the accuracy of measurement. Due to the high complexity of the interactions of all these factors, the training and its validation can finally usually only be done at the site and individually for each measurement pickup, this, in turn, leading to a considerable expense being associated with the start-up of the measurement pickup. Finally, it has also been found, that such classification algorithms, on the one hand because of the great complexity, and, on the other hand, because of the fact that, usually, a corresponding mathematical, physical model with technically relevant or understandable parameters is not explicitly present, classifiers exhibit a very low transparency and are, consequently, often difficult to place. Accompanying this, considerable resistance can arise with customers, with such acceptance problems especially occurring when it concerns classifiers involving a self-adapting mechanism, for instance a neural network.